In conventional slip casting, also known as colloidal filtration, molds are filled with slip, i.e. a suspension of fine solid particles in a liquid phase and the latter is removed from the suspension through the walls of the mold leaving the suspended particles behind on the walls. Fresh slip may be added to the mold to replenish the slip that has been removed and ensure that the mold remains full. For solid core casting, this process continues until the part is solid. For hollow core casting, the process continues until the desired wall thickness is achieved, at which point the slip remaining in the mold is poured or drained from the mold. In conventional casting, molds made of plaster of Paris are commonly used. The driving force for casting is the capillary pressure within the network of pore channels in the walls of the mold, the pore channels being of a size smaller than the suspended particles of the slip. As a cast layer of the filtered-out particles builds up on the mold walls, the cast layer itself acts as the filter and the particles continue to deposit.
Various factors affect the rate at which the process proceeds. The properties of the mold, of the suspension (slip) and of the cast layer are all important. For the normal casting of well-behaved slips of constant composition, the thickness of the cast layer is proportional to the square root of time. Various techniques have been used to increase the casting rate, for example pressure casting wherein a pressure is applied on the suspension, or vacuum casting in which the exterior of the mold is subjected to a vacuum. In the conventional casting, as mentioned above, the composition, microstructure and related properties of the cast layer and the resulting part (following the drying and thermal treatment of the cast layer) are uniform throughout.
Graded materials having a gradient in composition and, in some cases, in the microstructure (porosity content, grain size) hold potential for achieving higher performance levels than similar monolithic and composite materials in which the various phases are uniformly distributed. These graded materials are normally most appropriately utilized in applications for which the property requirements at opposite faces of a component differ. However, even in situations where both faces are subjected to similar conditions in service, compositional gradients may be used to enhance the performance. For example, symmetrical grading from both surfaces to the interior can be used to engineer materials containing residual compressive stresses at the surface. Such materials could have improved mechanical strength.
Various methods have been used to produce layered and graded bodies including: tape casting/lamination, see P. Boch et al., J. Am. Ceram. Soc., 69 (8) C-191-C-192 (1986); compaction of graded powders, see R. A. Cutler et al., J. Am. Ceram. Soc., 70 (10) 714-18 (1987), infiltration, see B. R. Marple et al., J. Mater. Sci., 28, 4637-43 (1993); sequential casting, see J. Requena et al. in Ceramic Transactions, Functionally Gradient Materials, Vol. 34, pp. 203-10, American Ceramic Society, Ed. J. B. Holt et al., 1993; electrophoretic deposition, see P. Sarkar et al., J. Am. Ceram. Soc., 75(10) 2907-909 (1992); and sedimentation-slip casting, see J. Chu et al., J. Ceram. Soc. Jpn., 101(7) 818-20 (1993). In some cases, these processes are suited for the production of only very simple geometries or limited to producing layered materials having a stepwise change in composition. The abrupt interface present between the zones in some layered or laminated materials may have a very positive effect on the behaviour of the material (for instance on the crack propagation). However, such interfaces are sometimes undesirable as may be the case where a difference in the coefficient of thermal expansion across the boundary leads to cracking. In these cases, a continuously graded material having a smooth transition in composition through the body may be preferred.
Accordingly, it is the object of the present invention to provide a process useful for making parts, usually ceramic parts or elements, with graded properties, rather than constant or abruptly changing properties, across the thickness of the part.
It is another object of the invention to provide an apparatus for making parts, especially ceramic parts, with graded properties as explained above.
It is still another object of the invention to provide a process and apparatus for making parts with continuously graded properties using the principle of slip casting.
It is yet another object of the invention to provide a controllable process and apparatus for making slip cast parts having predetermined properties.